A subject of the present invention is a process for preparation of fluorinated ketones. The invention is used more particularly for ketones having at least one fluorine atom, preferably three fluorine atoms at position xcex1 with respect to the carbonyl group.
It is known to prepare an xcex1-trifluorinated ketone which consists of reacting an organometallic compound with trifluoroacetic acid or its esters [Chem. L. S. et al, J. Fluorine Chem. VIII, p. 117 (1981)].
This process has several disadvantages. It includes several stages, preparation of the organometallic compound from bromobenzene, then reaction with trifluoracetic acid at a low temperature (xe2x88x9278xc2x0 C.) and hydrolysis, which complicates its implementation, and it is difficult to transfer to an industrial scale. Moreover, the reaction yield is not satisfactory due to the formation of by-products.
It is also known, in a general fashion, to prepare a ketone from one or more carboxylic acids according to the Piria reaction which consists of reacting the carboxylic acid or acids in the gaseous phase, in the presence of a metal oxide which can be chosen from the alkaline, alkaline-earth metal oxides, oxides of the metals of the groups IIIb, IVb and Vb.
It turns out that the preparation of a fluorinated ketone and more particularly an xcex1-trifluorinated ketone according to the Piria reaction has not been described in the literature, as defluorination of the starting reagent is carried out at high temperature, thus leading to fluorination of the catalyst.
A process has now been found, which constitutes the subject of the present invention, for preparation of a fluorinated ketone corresponding to the general formula: 
in which:
R1 and R2, which are identical or different, represent a hydrocarbon group containing 1 to 40 carbon atoms, which can be a linear or branched, saturated or unsaturated acyclic aliphatic group; a monocyclic or polycyclic, saturated, unsaturated or aromatic carbocyclic or heterocyclic group; a sequence of the above-mentioned groups.
at least one of the R1 and R2 groups does not comprise hydrogen atoms on the carbon atom at position xcex1 with respect to the carbonyl group,
at least one of the R1 and R2 groups comprises one or more fluorine atoms,
said process being characterized in that, in the gaseous phase,
a carboxylic acid of Formula (II): 
in which R1 is as defined above,
and a carboxylic acid of formula (III): 
in which R2 is as defined above,
is reacted in the presence of a catalyst comprising at least one oxide of an element chosen from the rare earths, thorium, titanium and aluminum.
By xe2x80x9crare earthxe2x80x9d is meant the lanthanides having an atomic number from 57 to 71, and yttrium as well as scandium.
According to the invention, carboxylic acids of Formulae (II) and (III) are divided, but the invention includes the use of carboxylic acid derivatives such as carboxylic acid anhydrides or corresponding ketenes.
The invention makes it possible to obtain symmetrical ketones if, in Formula (I), R1 is identical to R2, and asymmetrical ketones if R1 is different from R2.
More precisely, in Formulae (I) to (III), R1 and R2 represent a hydrocarbon group having 1 to 20 carbon atoms, which can be a linear or branched, saturated or unsaturated acyclic aliphatic group; a monocyclic or polycyclic, saturated, unsaturated or aromatic, carbocyclic or heterocyclic group; a linear or branched, saturated or unsaturated aliphatic group, carrying a cyclic substituent.
R1 and R2 preferably represent a linear or branched, saturated acyclic aliphatic group, preferably having 1 to 12 carbon atoms, and yet more preferably 1 to 4 carbon atoms.
The invention does not exclude the presence of an insaturation on the hydrocarbon chain, such as one or more double bonds which can be conjugated or non-conjugated, or a triple bond.
The hydrocarbon chain can optionally be interrupted by a heteroatom (for example oxygen or sulphur) or by a functional group to the extent that the latter does not react and a group such as in particular xe2x80x94COxe2x80x94 can in particular be mentioned.
The hydrocarbon chain can optionally carry one or more substituents (for example halogen, ester) to the extent that they do not interfere with the ketonization reaction.
The linear or branched, saturated or unsaturated acyclic aliphatic group can optionally carry a cyclic substituent. By ring is meant a saturated, unsaturated or aromatic, carbocyclic or heterocyclic ring.
The acyclic aliphatic group can be bonded to the ring by a valency bond, a heteroatom or a functional group such an oxy, carbonyl, carboxy, sulfonyl group, etc.
As examples of cyclic substituents, aromatic or heterocyclic, cycloaliphatic substituents can be envisaged, in particular cycloaliphatics comprising 6 carbon atoms in the ring, or benzenic, these cyclic substituents themselves optionally carrying any substituent to the extent that they do not impede the reactions intervening in the process of the invention. The alkyl, alkoxy groups having 1 to 4 carbon atoms can be mentioned in particular.
Among the aliphatic groups carrying a cyclic substituent, reference is made more particularly to the cycloalkylalkyl groups, for example cyclohexylalkyl, or the aralkyl groups having 7 to 12 carbon atoms, in particular benzyl or phenylethyl.
In Formulae (I) to (III), R1 and R2 can also represent a saturated or unsaturated carbocyclic group preferably having 5 or 6 carbon atoms in the ring; a saturated or unsaturated heterocyclic group, containing in particular 5 or 6 carbon atoms in the ring, including 1 or 2 heteroatoms such as nitrogen, sulphur and oxygen atoms; a monocyclic, aromatic carbocyclic or heterocyclic group, preferably phenyl, pyridyl, pyrazolyl, imidazolyl or polycyclic, condensed or non-condensed, preferably naphthyl.
As soon as one of the R1 and R2 groups comprises a ring, this can also be substituted. The substituent can be of any kind, to the extent that it does not interfere with the principal reaction. The number of substituents is generally 4 per ring at the most, but most often equal to 1 or 2.
Among all the meanings previously given for R1 and R2, they preferably represent a linear or branched alkyl group having 1 to 12 carbon atoms, preferably 1 to 6 carbon atoms or a phenyl group.
As previously mentioned, at least one of the R1 and R2 groups does not comprise hydrogen atoms on the carbon atom at position a with respect to the carbonyl group.
Therefore, one of the carbon atoms at position a with respect to the carbonyl group is a tertiary carbon atom. It can be represented by the formula (R3) (R4) (R5) Cxe2x80x94in which R3, R4, R5 in particular represent a halogen atom, preferably a fluorine atom; a linear or branched alkyl group having 1 to 6 carbon atoms, the R3, R4, R5 groups can also form a ring, for example a phenyl group.
As examples of carboxylic acids comprising a tertiary carbon atom, perfluorinated carboxylic acids can be mentioned.
In effect, the invention is used quite particularly for the preparation of fluorinated ketones from carboxylic acids, one of which is at least a fluorinated aliphatic carboxylic acid corresponding more particularly to Formula (IIa): 
in which:
Rf represents a perfluorinated chain of formula:
xe2x80x94[CF2]pxe2x80x94CF3
in said formula, p represents a number from 0 to 10.
The preferred aliphatic carboxylic acids correspond to Formula (IIa) in which Rf preferably represents the groups:
xe2x80x94CF3
xe2x80x94CF2xe2x80x94CF3
The invention is also used for the preparation of fluorinated ketones from fluorinated aromatic carboxylic acids, corresponding more particularly to Formula (IIb): 
in which Rf has the meaning given previously but preferably represents a trifluoromethyl group.
As examples of carboxylic acids of Formula (IIa) or Formula (IIb):
trifluoroacetic acid,
pentafluoroacetic acid,
the o-, m-and p-trifluoromethylbenzoic acids
can be mentioned.
As regards the carboxylic acid of Formula (III), those used preferably correspond to Formula (III) in which R2 represents:
a linear or branched alkyl group, having 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms,
a cycloalkyl group having 3 to 8 carbon atoms, preferably a cyclopentyl or cyclohexyl group,
an aryl group having 6 to 12 carbon atoms, preferably a phenyl or naphthyl group,
an alkylaryl group having 7 to 12 carbon atoms, preferably a benzyl group.
As examples of carboxylic acids of Formula (IIb),
acetic acid,
acetic anhydride,
propanoic acid,
butanoic acid,
pentanoic acid
can be mentioned more particularly.
According to the process of the invention, the carboxylic acid ketonization reaction is carried out in the presence of a catalyst comprising at least one oxide of an element chosen from the rare earths, thorium, titanium and aluminum.
This is more particularly an oxide of a rare earth chosen from the lanthanides, yttrium, scandium and their mixtures, preferably the lanthanides such as lanthanium, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
Mixtures of thorium oxide and oxides of rare earths are preferably used. More particularly, the mixtures of thorium oxide and cerium oxide are chosen.
Mixtures comprising 20 to 40% of thorium oxide and 60 to 80% of rare earth oxide, preferably cerium are advantageously used.
Another type of catalytic element suitable for the implementation of the process of the invention is aluminum.
All the commercial aluminas can be used, but use of gamma alumina is preferred.
An alumina having a specific surface of at least 100 m2/g is advantageously used, preferably between 150 and 350 m2/gm, and more preferably between 200 and 280 m2/g.
Its porous volume most often varies between 0.2 and 1.0 cm3/g, preferably between 0.4 and 0.9 cm3/g.
The porous volume, like the specific surface, is measured according to the BRUNEAU-EMMETT-TELLER method, described in the periodical xe2x80x9cThe Journal of American Society 60,309 (1938)xe2x80x9d.
Generally, the alumina is used in the form of extrudates having a diameter of 0.5 to 5 mm, preferably 1 to 3 mm and a length of 2 to 100 mm, preferably 5 to 10 mm. It is also possible for it to be in the form of beads, pellets or granules.
The active ingredient is provided either directly in the form of oxide or obtained after calcination of a support impregnated using a solution providing the above-mentioned elements in the form of hydroxide, mineral or organic, single or double salt.
It is possible to use a hydroxide, preferably a mineral salt, nitrate, sulphate, oxysulphate, halogenide, oxyhalogenide, silicate, carbonate, oxalate, or a preferred organic salt, acetylacetonate; alcoholate, and yet more preferably methylate or ethylate; C1-C6 carboxylate such as acetate or also carboxylates derived from C8-C22 fatty acids and in particular stearate, palmitate, myristate and laurate.
The active ingredient is advantageously used in oxide form.
The nature of the support can be variable. Therefore, in particular the active carbons, silica gels, silica-alumina mixtures, alumina, clays (and more particularly, kaolin, talc or montmorillonite), bauxite, magnesia, and diatomaceous earth can be suitable for the present invention.
In the catalyst, the content of active phase represents 5 to 100% of the weight of the catalyst.
The catalysts can be presented in different forms in the process of the invention: powder, moulded products such as granules (for example extrudates or beads), pellets, which are obtained by extrusion, moulding, compacting or any other type of known process.
In order to prepare the supported catalyst so that it can be used for implementation of the process of the present invention, it is possible to resort to standard techniques, known per se, for preparation of supported metal catalysts. Reference can be made, in particular, for the preparation of the different catalysts, to the work: [J. F. LEPAGE xe2x80x9cCatalyse de contactxe2x80x9d, conception, prxc3xa9paration et mise en oeuvre des catalyseurs industriels [xe2x80x9cContact catalysisxe2x80x9d, design, preparation and use of industrial catalysts], Edition Technip (1978)].
A known method for the preparation of metal oxides fixed on an inert support consists of dissolving in water a salt of the chosen metal, and pouring the solution obtained onto particles of the activated inert support. The mixture is then calcined between 400xc2x0 and 600xc2x0 C. in order to ensure the conversion of the metal salt to metal oxide fixed on the inert support.
According to the process of the invention, the ketonization reaction is carried out in the gaseous phase, by bringing the carboxylic acid (II) into contact with the carboxylic acid (III), in the presence of a catalyst as defined.
Generally, the quantity of the carboxylic acids used is such that the ratio between the number of moles of carboxylic acid (III) and the number of moles of carboxylic acid (II) varies between 1 and 20, preferably between 3 and 8.
According to the invention, the process is carried out in the gaseous phase. By this expression is meant that the different reagents are vaporized under reaction conditions, but the process does not exclude the presence of an optional liquid phase resulting either from the physical properties of the reagents, or from a use under pressure or the utilization of an organic solvent.
The vector gas is optional and is generally a gas or mixture of gases which are non-reactive under the reaction conditions. Gases such as nitrogen, air, argon or helium can be used. Advantageously, the volume ratio between the vector gas and the carboxylic acid (II) varies between 0 and 10, preferably between 0.1 and 2.0.
The temperature of the ketonization reaction is generally between 200xc2x0 C. and 500xc2x0 C., preferably between 250xc2x0 C. and 450xc2x0 C., and yet more preferably between 280xc2x0 C. and 300xc2x0 C.
The reaction pressure is preferably atmospheric pressure, but it is also possible to carry out the process under reduced pressure, which can go down to 100 mm of mercury when the starting reagents are not volatile.
According to the process of the invention, the starting reagents, i.e. the carboxylic acids, are vaporized. They are brought into contact with the catalyst, preferably carried along by a vector gas.
The WHSV (weight hourly space velocity) is between 0.05 and 1.5 hxe2x88x921, preferably between 0.1 and 0.8 hxe2x88x921.
In practice, the reaction is easily carried out continuously by passing the gaseous flux through a tubular reactor containing the catalyst.
First, the catalytic bed is prepared, which is constituted by the catalytic active phase, optionally deposited on a support (for example sintered glass), which allows the circulation of the gases without elution of the catalyst. Then, the reagents are introduced and several variants are possible.
It is possible to vaporize each of the reagents (II) and (III), in different chambers, then to carry out the mixing in a mixing chamber and to introduce the resultant gaseous flux onto the catalyst. The vector gas can be introduced in parallel with said gaseous flux or at the level of the mixing chamber.
Another variant consists of preparing a solution comprising the reagents (II) and (III), then vaporizing said mixture and introducing it onto the catalyst, in parallel with the vector gas.
Another practical embodiment of the invention consists of melting one of the carboxylic acids by heating it to its melting temperature and passing it over a gaseous flux comprising the other carboxylic acid. This flux is saturated with the first carboxylic acid and it is then brought into contact with the catalyst.
Another embodiment of the invention uses an organic solvent which is inert under the reaction conditions and which is chosen so that it solubilizes the carboxylic acid (II) and the carboxylic acid (III) used.
According to the invention, an aprotic solvent is preferably used, having a boiling point above 60xc2x0, preferably between 60xc2x0 C. and 300xc2x0 C.
As aprotic solvents capable of being used in the process of the invention, the aliphatic or aromatic hydrocarbons, such as hexane, heptane, cyclohexane, benzene, toluene, the xylenes can be mentioned.
Several solvents can also be used.
The quantity of carboxylic acid (II) introduced into the solvent is generally such that the solvent/carboxylic acid (II) molar ratio is between 0 and 20, and, preferably, between 0 and 5.
An organic solution is therefore prepared, comprising the carboxylic acid (II), the carboxylic acid (III), then said mixture is vaporized and introduced onto the catalyst, in parallel with the vector gas.
At the end of the reaction, the mixture of the gases is condensed and the non-reacted reagents and the products obtained are separated, by distillation or fractional crystallization. It is also possible to separate these by fractional condensation. In the more particular case of trifluoroacetone, the latter is recovered in hydrate form, by trapping the gaseous flux in water.
The process of the invention can make it possible to preferably obtain a ketone corresponding to Formula (I) of perfluorinated type.
The process of the invention is perfectly well adapted to the preparation of trifluoroacetone.
The process can be implemented continuously.
The preferred embodiment of the invention involves use of an alumina-type catalyst. The catalyst of the invention can easily be regenerated by air treatment between 450xc2x0 C. and 500xc2x0 C. It is maintained under air until no more carbon dioxide is released.
The regeneration of the catalyst can be carried out separately or in situ.
The regenerated catalyst retains all its catalytic performance.
The examples which follow illustrate the invention, without however limiting it.